1. Field of the Invention
The present invention relates to novel thermostable imido copolymers well adapted for the production of prepregs useful in the fabrication of multilayer circuits.
2. Description of the Prior Art
French Patent Application A-2,608,613 describes imido polymers and thermosetting prepolymers thereof, comprising the copolymerization reaction product, carried out at a temperature ranging from 50.degree. C. to 300.degree. C., of:
(a) one or more N,N'-bis-imides of the formula: ##STR1## in which the symbols Y, which may be identical or different, are each H, CH.sub.3 or Cl; and the symbol A is a divalent radical selected from among cyclohexylene, phenylenes, -4-methyl-1,3-phenylene, 2-methyl-1,3-phenylene, 5-methyl-1,3-phenylene and 2,5-diethyl-3-methyl-1,4-phenylene, or a radical of the formula: ##STR2## wherein T is a single valence bond or one of the groups: and the symbols X, which may be identical or different, are each a hydrogen atom or a methyl, ethyl or isopropyl radical;
(b) one or more sterically hindered diprimary diamines selected from among (i) the compounds of the general formula: ##STR3## wherein the symbols R.sub.1, R.sub.2, R.sub.3 and R.sub.4, which may be identical or different, are each a methyl, ethyl, propyl or isopropyl radical; and the symbols Z, which may be identical or different, are each a hydrogen atom or a chlorine atom; and (ii) the compounds of the general formula: ##STR4## wherein the amino radicals are in the meta or para position relative to each other; and the symbols R.sub.5, which may be identical or different, are each a methyl, ethyl, propyl or isopropyl radical;
(c) if appropriate, one or more non-halogenated monomers other than a bis-imide of formula (I) and comprising one or more polymerizable carbon-carbon double bonds; and
(d) if appropriate, a polymerization catalyst.
A primary advantage of the polymers described in the above '613 French patent application is a result of the steric hindrance of the amine reactant (b), which is responsible for the fact that the reactivity of the constituents of the polymerization medium is lower than in the case of using the polyamino-bis-maleimides derived from non-hindered diamines. This lower reactivity of the constituents of the polymerization medium is of particular interest not only in the production of moldings from a prepolymer in the molten state, but also in the production of moldings from a prepolymer in the form of a solution in a solvent. Indeed, the viscosity of the prepolymer in the molten state or that of the solution of the prepolymer changes less, which greatly facilitates the processing of the prepolymer, especially in applications requiring a working time on the order of several hours.
Another advantage of the polymers described in the '613 French patent application is that they are prepared without having to take particular precautions from the standpoint of health and hygiene, since the sterically hindered diamines (b) employed did not present the toxicity hazards generally associated with the non-hindered aromatic diprimary diamines.